The present invention is directed to a method and apparatus for producing oriented plastic strap. More particularly, the present invention is directed to a method and apparatus for milling and stretching a plastic sheet into strap stock material having a predetermined and consistent desired thickness.
Plastic strap is often used to package items in a bundle, on a pallet or in a crate for shipping, storage and merchandising. The strap is applied in a tensioned loop by an automatic or manually operated strapping machine. In a typical manufacturing process, a cast sheet of thermoplastic material, for example, polypropylene, is first reduced in size by rolling it through a pair of closely spaced milling rollers or cylinders that rotate in opposite directions. After the thickness of the sheet is reduced, the sheet is drawn and stretched out of the milling rollers by a series of orienting rollers or a bridle assembly to its final desired size.
One specific method that is commonly used is a process called the short gap method. An apparatus for carrying out this method includes an entry bridle, a stretching assembly and an exit bridle. A slow speed, heated bridle assembly advances a cast sheet of material, usually a film, to a stretching assembly. The stretching assembly includes a pair of rollers or cylinders set a distance apart. The first roller rotates at the same speed as the entry bridle. The second roller is rotating faster than the first roller and at the same speed as the exit bridle. Thus, as the film passes through the assembly, it is stretched to its final desired size.
Still another known method is called the zero gap method. In this method, the gap between the steps of milling and stretching are eliminated. Thus, the steps are carried out substantially simultaneously.
The zero gap method overcame many of the disadvantages of prior known methods, vis-à-vis limited increases in strength without significant decreases in other desired properties. In addition, the zero gap method reduced the necking that otherwise occurred as the sheet was stretched over the distance between the rollers.
It was however found that other concerns arose with use of the zero gap method. For example, it was noted that the lateral ends or sides of the sheet-strap tended to be thicker than the central sheet-strap regions. That is, there was less reduction in thickness or less flattening at the edges than in the middle or central region of the sheet-strap. Although the zero gap method in some respects addressed this problem by “pinning” the edges of the sheet-strap between the rollers, the lessened edge reduction nevertheless occurred.
In addition, it was noted that air tended to accumulate at the juncture of the strap or sheet and the roller. The accumulated or entrained air, which is generally “caught” by surface roughness or imperfections in the sheet-strap, moved within the apparatus, to between the sheet-strap and the milling rollers. This resulted in uneven thermal, e.g., heat transfer, properties across the lateral width of the roller/sheet interface.
It was also noted that in certain instances, the sheet-strap continued to be worked, e.g., elongated, even after it exited the last work roller. As a result, the amount of elongation of the sheet-strap (as during the working of the sheet-strap) was not as controlled as desired.
Accordingly, there is a need for an improved method for producing oriented strap. Such a method results in consistent strap thickness across the width of the strap. Desirably, in such a method, air accumulation at the first work roller is reduced or eliminated, resulting in consistent thermal and heat transfer properties at the strap/roller interface.